A coding technique for compressing audio signals at a low bit rate is a technique essential to realize the effective use of radio waves and so on in mobile communication. Meanwhile, there has recently been an increasing desire to improve audio quality in telephone communication, and implementation of telephone communication services that produce a greater sensation of presence is anticipated. To implement such services, audio signals having a wide frequency band at a high bit rate have to be coded. However, this approach conflicts with the effective use of radio waves and frequency bands.
Now, an audio signal coding technique adopted by Standard G.719 (ITU-T Standard G.719, 2008), for example, is studied.
In Standard G.719, upon coding an audio signal, a frequency transform is performed on the audio signal, and predetermined bits are allocated to a spectrum obtained as a result of the frequency transform. Specifically, the spectrum is divided into sub-bands having predetermined frequency bandwidths, and a unit (a unit having a needed number of bits) used in quantization based on lattice vector quantization is allocated to each of the sub-bands in decreasing order of energy as follows.
(1) One unit is allocated to a sub-band having the largest energy among all of the sub-bands.
One bit is allocated per spectrum. Therefore, if the number of spectral samples in a sub-band is eight, for example, one unit contains eight bits (note that the maximum number of bits that can be allocated per spectrum is nine bits, and therefore, if the number of spectral samples in a sub-frame is eight, up to 72 bits can be allocated).
(2) The quantized sub-band energy of the sub-band to which one unit has been allocated is decreased by two levels (6 dB). If the number of bits allocated to the sub-band to which one unit has been allocated exceeds the maximum value (nine bits), the sub-band is excluded from quantization in the succeeding loops.
Back to (1) above, the same process is repeated.
FIG. 6 illustrates the sub-band energy of each sub-band. The horizontal axis represents the frequency, and the vertical axis represents the amplitude on a logarithmic scale. In the figure, the sub-band energy of each sub-band is represented by a horizontal line instead of a point. The length of each horizontal line represents the frequency bandwidth of each sub-band.
FIG. 7 and FIG. 8 are diagrams illustrating examples of the results of bit allocation to each sub-band in a case of using a coding method specified in Standard G.719. In the figures, the horizontal axis represents the frequency, and the vertical axis represents the allocated number of bits. FIG. 7 illustrates a case of a bit rate of 128 kbit/s, and FIG. 8 illustrates a case of a bit rate of 64 kbit/s.
In the case of 128 kbit/s, an abundant bit budget is available for allocation, and therefore, nine bits, which is the maximum value, can be allocated to a large number of sub-bands (spectra), and the quality of audio signals can be maintained at a high level.
In contrast, in the case of 64 kbit/s, no sub-band is allocated nine bits, which is the maximum value, but every sub-band is allocated some bits. Accordingly, it is considered that degradation in the quality of audio signals can be suppressed and the effective use of radio waves and frequency bands can be realized.
However, the effective use of radio waves and frequency bands needs to be further promoted. Here, in a case of coding an audio signal having a sampling frequency of about 32 kHz at a low bit rate of 20 kbps/s or less by using the above-described method adopted by Standard G.719, it is not possible to reserve a unit (a number of bits) used in quantization of all sub-bands, which is a problem.
FIG. 9 is a diagram illustrating an example of the result of bit allocation to each sub-band in a case of using the coding method specified in Standard G.719 at 20 kbit/s. As illustrated, bit allocation fails not only in a high-frequency range but also, depending on the situation, in a low-frequency range, which is essential for hearing. Consequently, coding of spectra in the corresponding sub-bands is not possible, resulting in significant degradation in the quality of audio signals.
To solve such a problem, a method for dynamically changing a bit allocation method may be employed (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-534328).
However, the bit allocation method is changed while a single coding method (quantization method) is used without changing the coding method (quantization method), and therefore, this approach to degradation in the quality of audio signals has a limited effect.